High performance MOSFET with modulated channel gate thickness

ABSTRACT

A semiconductor device having gate oxide with a first thickness and a second thickness is formed by initially implanting a portion of the gate area of the semiconductor substrate with nitrogen ions and then forming a gate oxide on the gate area. Preferably the gate oxide is grown by exposing the gate area to an environment of oxygen. A nitrogen implant inhibits the rate of SiO 2  growth in an oxygen environment. Therefore, the portion of the gate area with implanted nitrogen atoms will grow or form a layer of gate oxide, such as SiO 2 , which is thinner than the portion of the gate area less heavily implanted or not implanted with nitrogen atoms. The gate oxide layer could be deposited rather than growing the gate oxide layer. After forming the gate oxide layer, polysilicon is deposited onto the gate oxide. The semiconductor substrate can then be implanted to form doped drain and source regions. Spacers can then be placed over the drain and source regions and adjacent the ends of the sidewalls of the gate.

FIELD OF THE INVENTION

The present invention relates generally to integrated circuitmanufacturing and more particularly to forming insulated gate fieldeffect transistors.

BACKGROUND OF THE INVENTION

An insulated-gate field-effect transistor (IGFET), such as a metal-oxidesemiconductor field-effect transistor (NOSFET), uses a gate to controlan underlying surface channel joining a source and a drain. The channel,source and drain are located in a semiconductor substrate, with thesource and drain being doped oppositely to the substrate. The gate isseparated from the semiconductor substrate by a thin insulating layersuch as a gate oxide. Currently, the gate oxide is formed having asubstantially uniform thickness. The operation of the IGFET involvesapplication of an input voltage to the gate, which sets up a transverseelectric field in the channel in order to modulate the longitudinalconductance of the channel.

In typical IGFET processing, the source and drain are formed byintroducing dopants of second conductivity type (P or N) into asemiconductor substrate of first conductivity type (N or P) using apatterned gate as a mask. This self-aligning procedure tends to improvepacking density and reduce parasitic overlap capacitances between thegate and the source and drain.

Polysilicon (also called polycrystalline silicon, poly-Si or poly) thinfilms have many important uses in IGFET technology. One of the keyinnovations is the use of heavily doped polysilicon in place of aluminumas the gate. Since polysilicon has the same high melting point as asilicon substrate, typically a blanket polysilicon layer is depositedprior to source and drain formation, and the polysilicon isanisotropically etched to provide a gate which provides a mask duringformation of the source and drain by ion implantation. Thereafter, adrive-in step is applied to repair crystalline damage and to drive-inand activate the implanted dopant.

There is a desire to reduce the dimensions of the IGFET. The impetus fordevice dimension reduction comes from several interests. One is thedesire to increase the number of individual IGFETs that can be placedonto a single silicon chip or die. More IGFETs on a single chip leads toincreased functionality. A second desire is to improve performance, andparticularly the speed, of the IGFET transistors. Increased speed allowsfor a greater number of operations to be performed in less time. IGFETsare used in great quantity in computers where the push to obtain higheroperation cycle speeds demands faster IGFET performance.

One method to increase the speed of an IGFET is to reduce the length ofthe conduction channel underneath the gate and dielectric layer regions.However, as IGFET dimensions are reduced and the supply voltage remainsconstant (e.g., 3 V), the electric field in the channel near the draintends to increase. If the electric field becomes strong enough, it cangive rise to so-called hot-carrier effects. For instance, hot electronscan overcome the potential energy barrier between the substrate and thegate insulator thereby causing hot carriers to become injected into thegate insulator. Trapped charge in the gate insulator due to injected hotcarriers accumulates over time and can lead to a permanent change in thethreshold voltage of the device.

A number of techniques have been utilized to reduce hot carrier effects.Several methods have been used in the past to form a graded dopingregion. One common technique for use with a typical gate having a gateoxide with a uniform thickness, is the formation of a graded doping inboth the source region and the drain region. The most common way to forma graded doping region is to form a lightly doped region in the drainwith a first ion implant using the sidewalls of a gate as aself-aligning mask. Spacers are then formed on the sidewalls of the gateand a second implant of dopant is made. In other words, the drain istypically formed by two ion implants. The first light implant isself-aligned to the gate, and a second heavy implant is self-aligned tothe gate on which sidewall spacers have been formed. The spacers aretypically oxides or nitrides. The part of the drain underneath thespacers is more lightly doped than the portion of the drain not shieldedby the spacers. This more lightly doped region is referred to as alightly doped drain (LDD).

The LDD reduces hot carrier effects by reducing the maximum lateralelectric field. The purpose of the lighter first dose is to form alightly doped region of the drain (or LDD) at the etch near the channel.The second heavier dose forms a low resistivity heavily doped region ofthe drain, which is subsequently merged with the lightly doped region.Since the heavily doped region is farther away from the channel than aconventional drain structure, the depth of the heavily doped region canbe made somewhat greater without adversely affecting the devicecharacteristics. The lightly doped region is not necessary for thesource (unless bidirectional current is used), however lightly dopedregions are typically formed for both the source and drain to avoidadditional processing steps.

As shown above, a threshold point exists where heightened speed andreduced dimensions will lead to IGFET breakdown. Conventional approacheshave encountered difficulty trying to reconcile the methods fordecreasing the hot carrier effects and the methods for improvingperformance. Also, it is desirable to achieve these sought after resultswithout adding costly processing steps. Thus, it is an objective touncover newly configured IGFET structures and the methods to produce thesame which will increase performance while not compromising the IGFET'slongevity or fabrication costs.

Graded-drain regions can be created in IGFETs in a number of ways,including: (1) using phosphorus in place of As as the dopant of thesource/drain regions; (2) adding fast diffusing phosphorus to anAs-doped drain region, and driving the phosphorus laterally ahead of thearsenic with a high temperature diffusion step to create adouble-diffused drain [DDD] structure; and (c) pulling the highly doped(n⁺) drain region away from the gate edge with an “oxide spacer” tocreate a lightly doped drain (LDD) structure. Each of these methodsrequires a number of processing steps. Most require two implant steps toform a lightly doped region and a heavily doped region. A method isneeded which reduces the number of implant processing steps.

SUMMARY OF THE INVENTION

A semiconductor device having gate oxide with a first thickness and asecond thickness is formed by initially implanting a portion of the gatearea of the semiconductor substrate with nitrogen ions and then forminga gate oxide on the gate area. Preferably the gate oxide is grown byexposing the gate area to an environment of oxygen. A nitrogen implantinhibits the rate of SiO₂ growth in an oxygen environment. Therefore,the portion of the gate area with implanted nitrogen atoms will grow orform a layer of gate oxide, such as SiO₂, which is thinner than theportion of the gate area less heavily implanted or not implanted withnitrogen atoms. The gate oxide layer could be deposited rather thangrowing the gate oxide layer. After forming the gate oxide layer,polysilicon is deposited onto the gate oxide. The semiconductorsubstrate can then be implanted to form doped drain and source regions.Spacers can then be placed over the drain and source regions andadjacent the ends of the sidewalls of the gate.

A method for forming a semiconductor device to produce graded doping inthe source region and the drain region includes the steps of implantingthe gate material, usually a polysilicon, with a dopant ion that variesthe level of oxide formation on the gate. The dopant ion is driven intoundoped polysilicon. Nitrogen ions, may also be implanted in thepolysilicon to contain the previously implanted ions. For N-typetransistors, typically arsenic is implanted. For P-type transistors,typically boron is implanted. Gates are formed. The gate structure isthen oxidized. The oxidation process is controlled to grow a desiredthickness of silicon dioxide on the gate. The portion of the gatecarrying the dopant grows silicon dioxide either more quickly or moreslowly. An isotropic etch can then be used to remove a portion of thesilicon oxide and form a knob on each sidewall of the gate. A heavy ionimplant is then done to convert a portion of the lightly doped sourceregion into a heavily doped region within the source region, and toconvert a portion of the lightly doped drain region into a heavily dopedregion within the drain region. Some of the implanted ions are stoppedby the knobs on the gate sidewalls. The regions under the knobs do nothave as deep an ion implantation resulting in a shallow region beneaththe knob. This forms a graded junction having a specific geometry. Thegeometry of the interface between the lightly doped region and theheavily doped region in the source region and the drain region dependson the geometry (thickness) of silicon dioxide knobs formed on thesidewall of the gate and on the length of the knob.

Advantageously, the dimensions of the silicon dioxide knob can be variedto form a graded channel having a different geometry. The steps areeasily performed and one implantation for heavy doping is all that isneeded to form the graded junction or doping pattern. The resultingdevice has a longer life, is more reliable and less likely to fail thandevices without graded doped drains and sources. In addition, thegeometry of the doping profile can be controlled more precisely usingthis invention. Information handling systems including such a device arealso more reliable and long lived.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the preferred embodiments can bestbe understood when read in conjunction with the following drawings, inwhich:

FIGS. 1A-1F show cross-sectional views of successive process steps formaking an IGFET having a uniform gate oxide layer and graded doping inthe drain region and source region.

FIGS. 2A-2I show cross-sectional views of successive process steps formaking an IGFET with a gate oxide having several thicknesses inaccordance with an embodiment of the invention.

FIG. 3 is a schematic of an information handling system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

An NMOSFET is described to show the most common method for forming atransistor device with a graded source and drain. In FIG. 1A, siliconsubstrate 102 suitable for integrated circuit manufacture includes aP-type epitaxial layer with a boron background concentration on theorder of 1×10¹⁶ atoms/cm³, a<100> orientation and a resistivity of 12ohm-cm. Preferably, the epitaxial surface layer is disposed on a P+ baselayer (not shown) and includes a planar top surface. Gate oxide 104,composed of silicon dioxide (SiO₂), is formed on the top surface ofsubstrate 102 using oxide tube growth at a temperature of 700° to 1000°C. in an O₂ containing ambient. A typical oxidation tube containsseveral sets of electronically powered heating coils surrounding thetube, which is either quartz, silicon carbide, or silicon. In O₂ gasoxidation, the wafers are placed in the tube in a quartz “boat” or“elephant” and the gas flow is directed across the wafer surfaces to theopposite or exhaust end of the tube. A gate oxide 104 having a uniformthickness is formed.

Thereafter, a blanket layer of undoped polysilicon 106 is deposited bylow pressure chemical vapor deposition (LPCVD) on the top surface ofgate oxide 104. Polysilicon 106 has a thickness of 2000 angstroms. Ifdesired, polysilicon 106 can be doped in situ as deposition occurs, ordoped before a subsequent etch step by implanting arsenic with a dosagein the range of 1×10¹⁵ to 5×10¹⁵ atoms/cm² and an energy in the range of2 to 80 kiloelectron-volts. However, it is generally preferred thatpolysilicon 106 be doped during an implantation step following asubsequent etch step.

In FIG. 1B, photoresist 110 is deposited as a continuous layer onpolysilicon 106 and selectively irradiated using a photolithographicsystem, such as a step and repeat optical projection system, in whichI-line ultraviolet light from a mercury-vapor lamp is projected througha first reticle and a focusing lens to obtain a first image pattern.Thereafter, the photoresist 110 is developed and the irradiated portionsof the photoresist are removed to provide openings in photoresist 110.The openings expose portions of polysilicon 106, thereby defining agate.

In FIG. 1C, an anisotropic etch is applied that removes the exposedportions of polysilicon 106. Various etchants can be used toanisotropically etch or to selectively remove the polysilicon and oxidelayers. Preferably, a first dry or plasma etch is applied that is highlyselective of polysilicon. Most of the polysilicon layer 106 is removed,except for the portion beneath the remaining photoresist 110. The gateoxide 104 is left on the surface of the silicon substrate 102 and has athickness in the range of 30-60 angstroms. Typically, the gate oxide 104is placed on the surface of the silicon substrate 102 at the selectedthickness in the range of 30-60 angstroms. Although unlikely, a seconddry or plasma etch may be applied that is highly selective of silicondioxide (the typical gate material), using the remaining photoresist 110as an etch mask to thin the layer of the gate oxide 104 to a selectedthickness. After the etching step or steps, a gate oxide layer of 30-60angstroms remains atop the surface of the silicon substrate, and theremaining portion of the polysilicon 106 provides polysilicon gate 112with opposing vertical sidewalls 114 and 116. Polysilicon gate 112 has alength (between sidewalls 114 and 116) of 3500 angstroms.

In FIG. 1D, photoresist 110 is stripped, and lightly doped source anddrain regions 120 and 122 are implanted into substrate 102 by subjectingthe structure to ion implantation of phosphorus, indicated by arrows124, at a dose in the range of 1×10¹³ to 5×10¹⁴ atoms/cm² and an energyin the range of 2 to 35 kiloelectron-volts. The ion implantation ofphosphorus is done through the layer of gate oxide 104. Polysilicon gate112 provides an implant mask for the underlying portion of substrate102. As a result, lightly doped source and drain regions 120 and 122 aresubstantially aligned with sidewalls 114 and 116, respectively. Lightlydoped source and drain regions 120 and 122 are doped N− with aphosphorus concentration in the range of about 1×10¹⁷ to 5×10¹⁸atoms/cm³.

As shown in FIG. 1E, spacers 132 and 134 are formed. A blanket layer ofsilicon dioxide with a thickness of approximately 2500 angstroms isconformably deposited over the exposed surfaces by CVD at a temperaturein the range of 300° C. to 400° C. Thereafter, the structure issubjected to an anisotropic etch, such as a reactive ion etch, that ishighly selective of silicon dioxide to form oxide spacers 132 and 134adjacent to sidewalls 114 and 116, respectively. Oxide spacers 132 and134 each extend approximately 1200 angstroms across substrate 102.

In FIG. 1F, the portions of the lightly doped source region 120 and thelightly doped drain region 122 outside oxide spacers 132 and 134 areconverted into heavily doped source region 150 and heavily doped drainregion 152 by subjecting the structure to ion implantation of arsenic,indicated by arrows 140, at a dose in the range of 2×10¹⁵ to 3×10¹⁵atoms/cm² and an energy in the range of 20 to 80 kiloelectron-volts.Polysilicon gate 112 and oxide spacers 132 and 134 provide an implantmask for the underlying portion of substrate 102. As a result, theheavily doped source region 150 and heavily doped drain region 152 aresubstantially aligned with the oxide spacer 132 on the side oppositesidewall 114, and the oxide spacer 134 on the side opposite sidewall116. A rapid thermal anneal on the order of 900° to 1050° C. for 10 to30 seconds is applied to remove crystalline damage and to drive-in andactivate the implanted dopants. As a result, heavily doped source region150 and the lightly doped source region 120 merge to form a source withgraded doping. Similarly, heavily doped source region 152 and thelightly doped source region 122 merge to form a drain with gradeddoping.

As shown in FIG. 2A, a substrate 102 has a field oxide layer 200deposited upon the substrate. Deposited on the field oxide layer 200 isa photoresist (not shown). The photoresist is masked, exposed and thenremoved. An etchant is placed in the removed area to form a gate area210 within the field oxide layer 200. The remaining photoresist iseither stripped or removed such that there is a first portion of fieldoxide layer 200 and a second portion of field oxide layer 200′. The areabetween the field oxide layers 200 and 200′ at the exposed substrate 102is the gate area 212.

As shown in FIG. 2B, thin oxide layer 214 is grown on the gate area 212.As shown in FIG. 2C the next step is to deposit nitride over the thinoxide layer and then to form nitride spacers from the deposited nitride.A first nitride spacer 232 is formed on one end of the gate area 212 anda second nitride spacer is formed on the other end of the gate area 212.Spacer 232 abuts the field oxide layer 200′. Spacer 234 abuts fieldoxide layer 200.

Now turning to FIG. 2D, nitrogen is implanted into the structure, asdepicted by arrows 240. The nitrogen implant is at a dose in the rangeof 1×10¹⁴ to 2×10¹⁵ atoms/cm² and at an energy in the range of 1-20kiloelectron-volts. Most importantly, the nitrogen is implanted into thesilicon layer 102, between the spacers 232 and 234. When nitrogen isimplanted into a silicon region, it serves to reduce the rate ofsubsequent oxide growth at the site of the implant, or in that region.

As shown in FIG. 2E, the nitride spacers 232 and 234 have been removed.In addition, an acid etch of hydrofloric acid HF with 10 parts water andone part acid is used as a wet etchant to remove oxide layer 214.

As shown in FIG. 2E, gate oxide 204, composed of silicon dioxide (SiO₂),is formed on the top surface of substrate 102 using oxide tube growth ata temperature of 700° to 1000° C. in an O₂ containing ambient. A typicaloxidation tube contains several sets of electronically powered heatingcoils surrounding the tube, which is either quartz, silicon carbide, orsilicon. In O₂ gas oxidation, the wafers are placed in the tube in aquartz “boat” or “elephant” and the gas flow is directed across thewafer surfaces to the opposite or exhaust end of the tube. The oxidelayer can also be formed using rapid thermal annealing (RTA). RTA hasseveral advantages over the use of an oxide tube, including less warpageof the wafers and localized heating.

The gate oxide 204 formed is not uniformly thick. The previous implantof nitrogen in the silicon base material 102 of the gate area 212inhibits the oxidation rate at the surface 212. In other words, theoxide layer 204 will grow slower in a silicon material that is dopedwith nitrogen when compared to a silicon material not doped withnitrogen. The nitrogen ions are only implanted in the area of the gatewhich was not covered by the spacers 232 and 234 in FIG. 2D. The rate ofoxidation in a silicon region not implanted with nitrogen grows fasterthan a silicon region implanted with nitrogen. The ratio of oxide growthfor the silicon region not implanted with nitrogen is in the range of1.5:1 to 3.4:1, when compared to the rate of oxidation in a nitrogenimplanted region. The end result is that the oxide layer 204 has a thinportion 244 and thick portion 246 and 248. The thick portions 246 and248 of the oxide layer 204 correspond to the portion of the substrate102 which was under the spacers 232 and 234. The thin portion 234 of theoxide layer 204 corresponds to the portion of the substrate 204 whichwas not under the spacers 232 and 234. Since the nitrogen is implantedbetween the spacers, the layer of oxide 244 is thinner than the portionsof the gated area 212 masked by the spacers 232 and 234. The oxide layer204 with the thin portion 244 and the thick portions 246 and 248 aregrown in a single process step.

Now turning to FIG. 2F, polysilicon 250 is deposited between the fieldoxide layer 200′ and the field oxide layer 200, and atop the oxide layer204. After the polysilicon 250 is deposited the top surface of thepolysilicon 250 and the oxide layers 200′ and 200 are polished to form asmooth surface.

Now turning to FIG. 2G, the oxide layers 200 and 200′ are removed usingan oxide etch. The oxide etch is very selective to the oxide layers 200and 200′ and can be either a dry or a wet etch. The resulting structureis a gate 260 having sidewalls 262 and 264. The next step is to implantarsenic ions to form a source and drain 272 and 274 (shown in FIG. 2H).The arsenic ion implantation, indicated by arrows 280, is at a dose inthe range of 2×10¹⁵ to 6×10¹⁵ atoms/cm² and at an energy in the range of10-80 kiloelectron-volts.

As shown in FIG. 2H, the spacers 292 and 294 are added to the sidewalls262 and 264. The spacers 292 and 294 are positioned over a portion ofthe source 272 and the drain 274. As shown in FIG. 21, the structure isthen subjected to a heat treatment such as an annealing process. The endresult is that some of the arsenic in the source 272 and drain 274migrates into some of the silicon substrate 102 underneath the oxidelayer 204. This forms a lightly doped region near the gate oxide 204,proximate each end of the gate oxide. Advantageously, only one implantstep is required. The channel width can be accurately controlled bycontrolling the width of the spacers 232 and 234. Although an NMOSFEThas been described above, a similar technique could be used to form aPMOSFET.

Further processing steps in the fabrication of IGFETs typically includeforming salicide contacts on the gate, source and drain, forming a thickoxide layer over the active region, forming contact windows in the oxidelayer to expose the salicide conforming interconnect metallization inthe contact windows, and forming a passivation layer over theinterconnect metallization. Salicidation includes the formation ofspacers on the gate, depositing a metal layer over the entire resultingsurface and reacting the metal to form a salicide on top of the gate112, on the top of the source 120 and on the top of the drain 122.Unreacted metal is then removed, glass is placed over the surface and acontact opening is formed for connectors. A passivation layer may alsothen deposited as a top surface. In addition, earlier or subsequenthigh-temperature process steps can be used to supplement or replace thedesired anneal, activation, and drive-in functions. These furtherprocessing steps are conventional and need not be repeated herein.Likewise the principal processing steps disclosed herein may be combinedwith other steps apparent to those skilled in the art.

The present invention includes numerous variations to the embodimentdescribed above. For instance, the gate insulator and spacers and can bevarious dielectrics including silicon dioxide, silicon nitride andsilicon oxynitride. Suitable N-type dopants include arsenic, phosphorusand combinations thereof. Alternatively, if a P-channel device isdesired, suitable P-type dopants include boron, boron species (such asboron difluoride) and combinations thereof

Advantageously, the invention is well-suited for use in a device such asan integrated circuit chip, as well as an electronic system including amicroprocessor, a memory and a system bus. The electronic system mayalso be an information handling system 500 as shown in FIG. 3. Theinformation handling system 500 includes a central processing unit 504,a random access memory 532, and a system bus 530 for communicativelycoupling the central processing unit 504 and the random access memory532. The information handling system 500 includes a device formed by thesteps shown in FIGS. 2A-2I, as described above. The system 500 may alsoinclude an input/output bus 510 and several devices peripheral devices,such as 512, 514, 516, 518, 520, and 522 may be attached to the inputoutput bus 510. Peripheral devices may include hard disk drives, floppydisk drives, monitors, keyboards and other such peripherals. Theinformation handling system 500 includes a device such as is shown inFIG. 2I. The channel formed as in the steps shown in FIGS. 2A-2I and theresulting device provides for a fast and reliable channel having a longlife. Faster channels are needed as clocking speeds for microprocessorsclimb and the channel must also be reliable and long-lived. The drainregions can be formed in one ion implant step rather than several. Thelength of the channel is also controllable since the spacers can also becontrolled.

Although specific embodiments have been illustrated and describedherein, it is appreciated by those of ordinary skill in the art that anyarrangement which is calculated to achieve the same purpose may besubstituted for the specific embodiments shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method for forming a device comprising thesteps of: providing a silicon substrate having a channel region; maskinga portion of the channel region with a nitride spacer and implanting aportion of the channel region of the silicon substrate with nitrogenions; and forming a gate oxide over the channel region.
 2. The method ofclaim 1 wherein the step of masking a portion of the channel region witha first nitride spacer located on one end of the channel region and asecond nitride spacer on another end of the channel region.
 3. Themethod of claim 2 wherein the step of forming a gate oxide on thechannel region includes removing at least one of the first and secondspacers from the channel region.
 4. The method of claim 2 wherein thestep of forming a gate oxide on the channel region includes removing thefirst and second spacers from the channel region.
 5. A method forforming a semiconductor device comprising the steps of: growing asilicon substrate; masking the silicon substrate to form an unmaskedchannel region of the silicon substrate; forming a thin oxide layer onthe unmasked channel region; masking a portion of the thin oxide layerto form an unmasked implantation region; implanting the channel regionbelow the unmasked implantation region with nitrogen ions, the nitrogenions being passed through the thin oxide layer; removing the thin oxidemask and the thin oxide; and forming a gate oxide over and above theunmasked channel region of the silicon substrate, the portion of thegate oxide over the implanted silicon being thinner than the gate oxidenot formed over the implanted silicon.
 6. The method of claim 5, whereinforming the gate oxide over and above the unmasked channel regionincludes forming the gate oxide on the substrate.
 7. The method of claim5, further comprising forming a gate over the gate oxide, the gate widthbeing defined by the width of the unmasked channel region and beingabout equal in width to the channel region.
 8. The method of claim 7,further comprising: removing the silicon substrate mask; and doping theportion of the silicon substrate adjacent the gate and the channelregion to form source/drain regions.
 9. The method of claim 8, whereindoping the portion of the silicon substrate adjacent the gate andchannel region to form source/drain regions includes formingsource/drain regions that are in the same horizontal plane as andadjacent the nitrogen-implanted channel region.
 10. The method of claim8, wherein the source/drain regions are not formed below thenitrogen-implanted channel region.